This invention relates to a device and a process for determining the physical parameters of a two-phase mix by propagating an acoustic wave in the continuous phase of the mix. For example, the invention can be used to determine the propagation time of an acoustic wave in the continuous phase of the two-phase mix. In particular, the device can be used to measure the acoustic impedance of this phase, its density and the propagation velocity of the acoustic wave in the continuous phase.
For the purposes of the invention, a two-phase mix means any emulsion or dispersion in which a first solution or first phase is in the form of a continuous phase, and a second solid, liquid or gaseous phase is in the form of droplets or particles dispersed in the continuous phase. The second phase is called the dispersed phase.
This type of two-phase mix is used particularly to separate chemical elements in solution. The separation process consists essentially of putting a first solution containing chemical elements into contact with a second solution that acts as an extractor. Putting them into contact in this way enables transfer of material between the two solutions.
The transfer of material is facilitated by the formation of a two-phase mix in the form of an emulsion or dispersion of fine droplets. Settlement finally separates the liquids after the material has been transferred.
Different separation devices operating according to the process mentioned above are known. These include mixer-decanter type devices, centrifugal extractor devices, or pulsed column devices.
The invention may be used to analyze the physical or chemical properties of a two-phase mix, and in particular is used for monitoring parameters essential for such an analysis, namely firstly the propagation velocity of an acoustic wave in the continuous phase of the mix and secondly its density. A change in the propagation velocity is recorded whenever the density of the continuous phase is modified. For example, this type of change in the density corresponds to a transfer of material between the phases.
The invention is used for applications in the oil, pharmaceutical, chemical and food processing industries, in the treatment of radioactive waste and more generally in any field in which an emulsion needs to be characterized.
When a separation process is used, the two-phase mix is usually monitored, particularly by determining the proportion of each phase in the mix.
Document (1), referenced at the end of the description, describes in particular a process for measurement of the fraction by volume of one of the phases in a two-phase mix consisting of a first phase F1 kept inside a receptacle, in the dispersed state in a second phase F2. According to this process, the propagation velocity v1 of an ultrasound signal in phase F1, and the propagation velocity v2 of an ultrasound signal in phase F2 are determined, and an ultrasound signal is then emitted at a point P1 in the said receptacle, the passage of the said signal at a point P2 in the said receptacle located at distance d from point P1 is detected, the time t spent by the said signal to travel the distance d is determined, and the fraction by volume xcex51 of phase F1 and/or the fraction by volume xcex52 of phase F2 are calculated using the following equations:             ϵ      1        =                                        v            1                    ⅆ                ·                                                            v                2                            ⁢              t                        -            ⅆ                                              v              2                        -                          v              1                                          ⁢              xe2x80x83            ⁢      and                  ϵ      2        =                            v          2                ⅆ            ·                                                  v              1                        ⁢            t                    -          ⅆ                                      v            1                    -                      v            2                              
Document (2), also referenced at the end of this description, relates to a similar process also using a method for measuring the propagation time for an acoustic wave in a two-phase mix.
The first step in calculating the fraction by volume of either of the phases, is to determine the propagation velocities (v1, v2) of the acoustic wave in each phase, as is the case in document (1).
Thus, document (2) describes how to measure the wave propagation velocity in each of the liquids to be mixed, before making the mix.
One difficulty in measuring the fraction by volume of either of the phases is due to the fact that the propagation velocities or propagation times are not constant during the separation. These parameters are influenced by temperature modifications, but also by changes to the density of the phases related to the material transfer.
It is not very difficult to determine the change in propagation velocities and times as a function of the temperature.
There are two possible methods at the moment of allowing for the change in propagation velocities or times as a function of material exchanges from one phase to the other.
The first method is to take samples of a small volume of emulsion during the separation treatment, and to make acoustic propagation velocity or time measurements separately in each phase after allowing them to settle.
However, there are disadvantages with this first method. Taking the sample of emulsion can disturb operation of the separation device. Furthermore, sampling is only possible if the separation device contains a sufficiently large volume of the mix. Moreover, the sampled volume must be reinjected into the separation device or must be stored after each measurement.
Finally, in the case in which the two-phase mix contains strongly radioactive bodies, it may be impossible to extract and store measurement samples.
A second method of determining the wave propagation velocities or times in each of the phases separately during the treatment is to form settlement chambers in the separation device adjacent to a mixing area. These xe2x80x9cin situxe2x80x9d settlement chambers can modify the hydraulic behavior of the device and make local modifications to the characteristics of the two-phase mix.
These devices are also used for on-line measurement of the density of the continuous phase.
The purpose of this invention is to propose a process and a device for determining physical parameters such as the propagation velocity of an acoustic wave in the continuous phase of a two-phase mix, the acoustic impedance of the continuous phase and/or its density, and with none of the difficulties mentioned above.
One purpose in particular is to enable this type of continuous measurements to be taken without interrupting the separation process and without taking any samples of the two-phase mix.
Another purpose is to propose a non-intrusive device and process that have no influence on the hydraulic operation of separation equipment and that do not modify the characteristics of the two-phase mix.
In order to achieve these purposes, the purpose of the invention is more precisely a device for measuring the propagation time of an acoustic wave in a continuous phase of a two phase mix, the device comprising an electro-acoustic transducer capable of emitting acoustic waves and outputting a reflected acoustic wave reception signal, and means of using transducer signals to determine the time necessary for propagation of waves from the signals output by the transducer. The device also comprises means of focusing the acoustic waves in a focusing area and the frequency of the acoustic waves is adjusted to reflect the waves on the droplets of the dispersed phase, located approximately in the focusing area.
The wavelength xcex of the acoustic wave produced by the electro-acoustic transducer is such that:   λ  =            v      c        f  
In this equation, Vc represents the wave propagation velocity in the continuous phase and f is the wave frequency.
A droplet of the dispersed phase located within the focusing area causes a local disturbance of the acoustic impedance of the medium through which the wave passes at this location. The acoustic impedance of a medium is defined in this case as the product of its density and the wave propagation velocity in the medium.
The acoustic wave may be reflected by a droplet of the dispersed phase present in the focusing area and the energy of the reflected acoustic signal is proportional to the difference between the acoustic impedances of the dispersed phase and the continuous phase. Efficient reflection is obtained when the diameter of the droplets is greater than the wavelength xcex.
Thus, starting from an estimate of the velocity Vc, the minimum frequency can be determined as a function of the diameter xcfx86 of the smallest droplets present significantly in the two-phase mix. For example, the frequency is adjusted to fix the wavelength xcex such that xcex=xcfx86/5.
The use of an ultrasound transducer operating at a high frequency, for example above 100 MHz, guarantees that reflections will occur on the droplets even if there is a large change in the two-phase mix. The size of the droplets in a separation device changes as a function of the operating conditions and the retention rate of the continuous phase.
The focusing distance of the acoustic wave is preferably chosen as a compromise between the minimum droplet diameter that can be measured for a given frequency and attenuation of the wave in the continuous phase. This means that the propagation time can be determined with a good precision with a reasonable probability of passing through the focusing area of a droplet during a measurement time, taking account of an average distance between the droplets.
For example, the focusing distance may be of the order of one millimeter.
The frequency of the acoustic wave may also be optimized as a function of the attenuation of the medium.
The propagation time in the continuous phase of a wave that is reflected on a droplet located in the focusing area corresponds to the time taken to travel twice the focal distance of the focusing means.
Thus, means for using the signals output by the device according to the invention can be provided to determine the wave propagation velocity Vc in the continuous phase according to the equation       v    c    =            2      ⁢              xe2x80x83            ⁢      F        T  
In this equation, F is the focal distance of the focusing means and T is the propagation time.
For example, the focusing means may comprise an acoustic lens with a first face on which the electro-acoustic transducer is fitted, and a second face with at least one concave portion with a radius of curvature R, facing the mix, and called the emission face. This emission face is designed to focus acoustic waves towards the focusing area. When an acoustic wave is emitted, a first partial reflection takes place on the emission face, and then a second series of reflections may also take place on a droplet inside the focusing area.
Thus, according to one particular aspect of the invention, the means of using the signals may be designed to set up a delay time between a first reflection signal on the emission face of the lens and a second reflection signal on a droplet of the dispersed phase, in response to the same emitted acoustic wave. The propagation time for the wave that travels twice the focal distance F of the lens is then equal to this delay time.
The propagation velocity of the acoustic wave can then be calculated from the focal distance F according to the equation       v    c    =            2      ⁢              xe2x80x83            ⁢      F        T  
indicated above, or as a paraxial approximation starting from the radius of curvature R of the emission face and the propagation velocity Vv in the lens material.
The result is   F  =            R              1        -                              V            c                    /                      V            v                                =                            V          c                ·        T            2      
Thus, Vc2xe2x88x92Vc.Vv+2.R.Vv/T=0.
Vc can be calculated from this equation (in glass, Vv≈5968 m.sxe2x88x921).
The amplitude of the signal output by the transducer is greater when the acoustic impedance of the droplet is not the same as the acoustic impedance of the liquid forming the continuous phase. As a first approximation, it is assumed that the droplet can be represented by a plane disk that passes in front of the focusing area. It is assumed that the dimension of the droplet is greater than the acoustic wavelength. In the most favorable case, the collected signal can be modeled by the function       V    ⁡          (      z      )        =            ∫      0              θ        0              ⁢                  R        ⁡                  (          θ          )                    ⁢              P        ⁡                  (          θ          )                    ⁢      cos      ⁢              xe2x80x83            ⁢      θ      ⁢              xe2x80x83            ⁢              ⅇ                  kz          ⁢                      xe2x80x83                    ⁢          cos          ⁢                      xe2x80x83                    ⁢          θ                    ⁢              ⅆ        θ            
in which V(z) represents the output signal when the droplet moves along an axis of the acoustic lens by a distance z, R(xcex8) is the reflection power of the droplet, xcex8 is the angle of incidence at which the acoustic wave reaches the droplet, P(xcex8) is a function corresponding to the opening of the lens (pupil function), k is the wave vector in the continuous medium and xcex80 is the half-opening angle of the lens (≈50xc2x0). The maximum signal amplitude is obtained when the droplet is in or close to the focal plane. This characteristic can be used to accurately determine the flight time T between the emission face of the lens and the focusing area. The propagation time is preferably determined based on a large number of passes through droplets near the focal point.
According to one particular embodiment of the acoustic lens, its emission face may comprise at least one concave portion capable of focusing acoustic waves, and a plane portion capable of reflecting non-focused waves.
It is not essential to make the emission face with a plane part, but it does make it possible to make measurements of reflections of waves riot focused on this part more accurately. This point is described in more detail in the rest of the text.
According to one improvement, the emission face of the acoustic lens may also be covered with a thin coat of anti-reflection material with a thickness approximately equal to xcex0/4, where xcex0 is the acoustic wavelength. Furthermore, the acoustic impedance Za of the coat of anti-reflection material may also be chosen equal to Za={square root over (ZL.ZC)}, where ZL is the acoustic impedance of the material (for example glass) from which the acoustic lens is made, and ZC is the estimated impedance of the continuous phase.
The anti-reflection coating performs two functions, firstly it improves the determination of a reflection power of the continuous phase (described later) by the plane portion of the emission face, and secondly it corrects convergence astigmatism of the concave part of the emission face.
Note that the use of an acoustic lens is not the only possible means of focusing the acoustic wave. Thus, as a variant, the transducer may be a segmented type transducer comprising a plurality of transducer elements (sensors) that can be excited separately. These elements, individually controlled by adapted electronic circuits, are used to directly focus the acoustic wave produced. They can thus form focusing means according to the meaning of the invention.
According to one particular application of the invention, the process can be used to measure the propagation velocity of an acoustic wave in the continuous phase of a two-phase mix comprising the continuous phase and a dispersed phase forming droplets in the continuous phase. According to this process:
acoustic waves focused in a focusing area are emitted in the continuous phase, the frequency of the acoustic wave being adjusted to enable a reflection on droplets in the dispersed phase located approximately in the focusing area,
the first reflection signals from waves on the droplets are recorded,
a propagation time for the wave is determined from the reflection signals, and
the propagation velocity is calculated starting from the propagation time and an acoustic wave focusing distance.
According to another particular application of the invention, the reflection signals may also be used to determine the reflection power of the continuous phase. This reflection power is defined as the ratio of the amplitude of the reflected signals to the amplitude of the emitted acoustic signals.
In this respect, the second reflection signals from an acoustic wave reflected at an interface between the acoustic lens of the electro-acoustic transducer and the continuous phase, can be recorded.
Preferably, reflection signals from the plane portion of the emission face corresponding to a non-focused wave can be used to determine the reflection power Rc.
For example, the signal amplitude can be recorded and measured using a simple oscilloscope.
The reflection power Rc of the continuous phase can be determined directly from the signal amplitude, for example as a function of a predetermined (approximately linear) calibration curve.
The calibration curve may be determined by dipping the lens in the fluids for which the reflection power is known, for example such as air or water, and measuring the amplitude of the waves reflected by these fluids.
According to another possibility, the reflection power of the continuous phase may be determined as the ratio of the amplitude of signals reflected from the continuous phase, to the amplitude of the emitted signals.
The amplitude and therefore the energy of the emitted signals may be known starting from an additional and partial reflection of the signals on a reference diopter. This aspect will be dealt with in more detail in the rest of the description.
According to another aspect of the invention, it is also possible to establish an acoustic impedance Zc of the continuous phase, if the reflection power Rc of the continuous phase is known, by using the following equation:             Z      C        =                  Z        L            ⁢                        1          -                      R            C                                    1          +                      R            C                                ,
where ZL is an acoustic impedance (known) of the material from which the acoustic lens is made.
When an anti-reflection coat with acoustic impedance Za is provided on the acoustic lens, the result is       Z    C    2    =            Z      L        ·          Z      a        ·                  1        -                  R          C                            1        +                  R          C                    
According to another aspect of the invention, the density xcfx81c of the continuous phase can also be established from the following equation:       ρ    ⁢          xe2x80x83        ⁢    c    =            Z      C              V      C      
The reflection power RD between the continuous phase and the dispersed phase can also be determined from a measurement of the maximum amplitude of the said first reflection signals, in other words signals corresponding to a reflection on the droplets of the dispersed phase.
In this case, the value of RD is also obtained by comparing the amplitude of the first reflection signals with a pre-determined calibration curve, using dispersions or media with known properties.
Finally, knowing the acoustic impedance of the continuous phase and the reflection power RD, the acoustic impedance ZD of the dispersed phase can be calculated using the following equation:       Z    D    =            Z      C        ⁢                  1        -                  R          D                            1        +                  R          D                    
In this equation, Zc is the impedance of the continuous phase determined in advance as described above.
Other characteristics and advantages of this invention will become clearer from the following description given with reference to the figures in the attached drawings. This description is given for illustrative purposes only and is in no way limitative.